1. Field of the Invention
The present invention generally relates to a communications system, and more particularly to a system and method for performing an error control operation in a radio communications system.
2. Description of the Background Art
FIG. 1 shows a receiver in a conventional wireless communications system. This receiver includes an MPSK demodulator 10 for demodulating a re-transmitted packet signal using a soft decision method, a diversity combiner 20 for combining a symbol generated from the demodulator 10 with a previously received error packet signal (hereinafter referred to as ‘original packet signal’), and a decoder 30 for decoding the combined symbol.
The MPSK (M-ary Phase Shift Keying) demodulator performs a modulation method in which only phase is changed with amplitude, a frequency of a carrier is constantly maintained, and a plurality of bits are transmitted at one time. As the signal is modulated, 2-bit or 4-bit data is transmitted by one modulation signal. That is, an unmodulated signal is transmitted one bit at a time but in the case of 4 binary-PSK modulations 2 bits are transmitted at a time using a phase change of the signal. 16 binary-PSK allows transmission of 4 bits at a time.
MPSK modulation is based on the assumption that M=2n where M is the number of modulation signals (or symbols) with different phases and n signifies the number if bits transmittable at a time. In this modulation scheme, a phase difference between carriers is 2π/M and can be implemented using an orthogonal representation system.
The diversity combiner performs a symbol diversity combining technique. According to this technique, symbols with noise are held, not discarded, and are then effectively combined with the same symbols which have been re-transmitted in order to control an error and thereby heighten reliability of the symbols. Adoption of a symbol diversity combining method in a radio communication system is desirable because it increases reliability of the symbols. Thus, various diversity combining methods have been proposed such as an equal-gaining-combining method and a maximal-ratio-combining, method.
Among these, the maximal-ratio-combining method is considered the most effective. This method adds ratios of the sizes of signals and then maximizes a signal-to-noise ratio in an environment where a signal plus an additive white Gaussian noise is received. The maximal-ratio-combining method has excellent performance but its calculation method is very complicated because it uses a soft-decision value.
In operation, the signal output from the MPSK demodulator (i.e. the symbol) is a value obtained by performing a soft-decision operation on a re-transmitted packet signal as received. This value is generally expressed as a real number with a decimal point. When the symbol is transmitted to the diversity combiner 20, the diversity combiner combines it with a symbol of an original packet. The combining process performed by the diversity combiner is expressed as follows:(weight value x*symbol A)+(weight value y*symbol B)→(1.1*0.789)+(0.8*1.125)=1.768wherein the weight value is a weight value of a channel which transmitted an original packet, the weight value ‘Y’ (0.8) is a weight value of a channel which transmitted a re-transmission packet, the symbol ‘A’ is a symbol of an original packet, the symbol ‘B’ (1,125) is a symbol of a re-transmission packet, and 1.768 is a result value output from the diversity combining process according to the MRC.
The result value (1.768) of the combining process is transmitted to the decoder 20, and the decoder determines the re-transmitted packet signal using the received result value and performing error-correction decoding. The decoder 30 usually performs only an error-correction function. Though not shown in Figure 1, the maximal-ratio combining (MRC) diversity combiner includes a channel estimator to overcome multipath fading. The channel estimator attempts to estimate a size of a channel through which received signals and phase information have passed.
The result value (1.768) of the combining process is transmitted to the decoder 20, and the decoder determines the re-transmitted packet signal using the received result value and performing error-correction decoding. The decoder 30 usually performs only an error-correction function. Though not shown in FIG. 1, the MRC diversity combiner includes a channel estimator to overcome multipath fading. The channel estimator attempts to estimate a size of a channel through which received signals and phase information have passed.
The MRC diversity combiner of the conventional art has disadvantages. For example, this MRC requires use of the channel estimator. Also, this MRC is hard to implement due to use of the soft decision value in the symbol demodulation process and requires a very large amount of calculations to be performed, which translates into increased complexity. These calculations include multiplication and addition of real numbers below a decimal point and are performed in the MRC scores of times more than the bit logical operation of the present invention, described in detail below. The conventional art MRC is therefore very complex. In addition, in the MRC a channel estimated value, a soft-decision value, and a calculation result value are stored as real numbers. As a result memory space is wasted.
A need therefore exists for a diversity combiner which performs fewer calculations that a conventional MRC combiner and which is therefore more efficient and cost effective to implement.